There is a need for efficient storage of thermal energy within the area of modern energy technology.
Thermal energy may advantageously be stored in a fluid, such as e.g. water, above ground in insulated tanks, in ground in insulated pits, or underground in excavated caverns, using the surrounding ground as insulation. The thermal energy of the fluid is preserved to a great extent during an extended period of time. Today, these methods are used in different parts of the world in order to satisfy the need for storing thermal energy between different seasons, e.g. storing temporary surplus heat which is used later on when there is a demand for it and, preferably, when its financial value is higher. The main transition of energy is from the summer half, when there is less need for heating, to the winter half, when the need for heating is much higher. However, there is also much to gain by using the storage for short-term variations and always actively storing surplus heat. These kinds of storages may also be used for storage of a colder fluid, to be used for cooling, as well as for fluid having an intermediate temperature, such as a fluid used in low temperature systems.
As mentioned above, it is a common solution to store thermal energy in a fluid located in a cavern, utilizing the surrounding ground as insulation. This has the advantages of a large storage capacity per unit volume and the possibility of withdrawing a large output from the storage. Hence, this kind of storage is suitable both for short and long term storage. However, there are also substantial disadvantages such as high investment costs.
A further solution is to use a storage comprising a large number of vertically extending and evenly distributed channels. Hot water is circulated through the channels, and the ground itself stores the thermal energy. This has the advantages of low investment costs and the fact that it can be used in ground of differing quality. However, the slow speed for transporting heat through the ground is a huge disadvantage, since high storage and withdrawal speeds are impossible. Hence, this kind of storage is mostly suitable for long term storage, i.e. storage between seasons. The thermal losses to the surrounding ground and the atmosphere are in this case considerable due to a relatively large surrounding area compared to the enclosed storage volume. In this type of storage, pipes and pump arrangements for circulation of liquid are installed for injection and retrieval of energy. A considerable amount of additional electric energy is therefore required for these operations, reducing the efficiency of the storage substantially.
Swedish patent application 0950576-9 discloses one kind of efficient storage of thermal energy. However, there is still a need for an even more improved arrangement for storing thermal energy underground.